I have a stock 2008 YZ450 with 26 hours on it. I use Amsoil MCF and change it every five hours. I have not been into the motor. Recently I have been getting some loud ticking, almost a clacking, out of the top end. It comes and goes, almost like something is stuck and then frees up. Yesterday (Sunday) I started it and there was no noise. I let it warm up for a minute. As soon as I gave it some gas I got the sound of really loose valves. If I rev the motor just a few thousand rpms it fades, then I hear it again at idle. I went out for practice and when I came back, no more noise. No more noise for the rest of the day. It did the same thing last weekend but I also rode Saturday with no occurrence. I am hoping someone might shed some light on this before I go digging. Any input is appreciated.